The desired value that the attribute must have.
If this is null, the validator will validate that the specified attribute is not empty.
If this is set as a value that is not null, the validator will validate that
the attribute has a value that is the same as this property value.
Defaults to null.

Whether the comparison between the attribute value and $requiredValue is strict.
When this is true, both the values and types must match.
Defaults to false, meaning only the values need to match.
Note that when $requiredValue is null, if this property is true, the validator will check
if the attribute value is null; If this property is false, the validator will call isEmpty()
to check if the attribute value is empty.